What happened
On June 3, 2007, a Boeing B737-800, operated by Ryanair Ltd. as flight RYR 586A, was descending through flight level 370 toward its destination of Pisa. The aircraft was transitioning through Swiss airspace, with instructions to reach flight level 290 by the ODINA waypoint. Simultaneously, a Cessna 560 Citation V Ultra, registered HB-VNW and operated by Swiss Eagle AG as flight EAB 627, was cruising at flight level 300, traveling from Toulon-Hyères to Zurich.
As the Ryanair flight descended, the air traffic controller in the Zurich sector instructed the crew to contact Milan ATC. Upon contacting Milan, the crew was directed to initiate a left turn toward the Parma VOR. This maneuver placed the Boeing B737-800 on a head-on course with the HB-VNW.
The investigation
The investigation examined the coordination between the Swiss Radar (Zurich) and Milano Radar (Milan) sectors. It was established that while the Zurich controller had coordinated a descent to flight level 290, the subsequent instruction from Milan to turn toward Parma was issued without coordinating the new heading with the Zurich controller.
Radar records showed that a Short Term Conflict Alert (STCA) was triggered at 15:49:46 UTC. The aircraft eventually passed within 2.3 nautical miles of each other with a vertical separation of only 650 feet. The investigation also looked into the use of the Traffic Alert and Collision Avoidance System (TCAS) by both crews. While the crew of the HB-VNW initially reported a Resolution Advisory (RA) to the controller, they later clarified during interviews that they had only received a Traffic Advisory (TA).
Findings
- The primary cause of the near-collision was the lack of coordination between the Milan and Zurich air traffic control sectors regarding the new heading assigned to the Ryanair flight.
- The Milan controller issued a turn instruction to the Boeing B747-800 without verifying if the new course would conflict with existing traffic in the adjacent sector.
- The Ryanair crew's descent rate decreased after initiating the turn, which contributed to the closing geometry with the oncoming aircraft.
- There was a lack of proficiency in the HB-VNW crew regarding the technical distinction between TCAS Traffic Advisories and Resolution Advisories.